The long term goals of this project are to define the functions of tight junctions in Stria vascularis of the inner ear and to determine if the diversity of Claudin family members that comprise fight junctions located around Scala media have overlapping or non-overlapping properties. The knowledge gained from these studies will provide deep insights into the roles of the stria vascularis in hearing and potassium recycling to the endolymph. The specific aims are: 1) to test the hypothesis that inner ear pathology in Claudin//-null mice stems from morphological defects. Claudin 11 is expressed in the developing vestibulocochlear apparatus of embryos from E13.5 as well as in adults and evidence of gross structural changes during development and postnatally that could account for the pathology in the knockout mice will be sought. 2) to test the hypothesis that Claudin 11-null mice exhibit early hearing defects. Young adult knockout mice exhibit elevated auditory brainstem response (ABR) thresholds which indicate hearing loss, Further testing of ABRs as well as measurements of endocochlear potentials and DPOAEs at different ages will reveal if this phenotype is progressive or present at the time mice normally start to hear. 3) to optimize a transgene cassette for the expression of heterologous genes in basal cells of Stria vascularis. Basal cell-specific enhancer elements already in-hand will be located within the Claudin 11 gene and used to drive expression of a beta-galactosidase reporter gene in the cochlea, Reporter expression will be characterized with a view to expressing several claudin cDNAs in the cochlea in attempts to rescue hearing loss in Claudin 11-null mice.